The invention relates to a structure for shielding a stray light in an optical waveguide module, and especially to a structure for shielding a stray light leaked from a laser light to a photodiode for receiving a signal light.
Recently, a field of an application of an optical communication is rapidly shifting to a subscriber line from a trunk line. It is the general trend that optical modules used in optical subscriber systems are optical transceivers which have functions of transmitting and receiving signal lights simultaneously on the basis of introduction of the optical waveguides etc. for miniaturization and economization.
In the conventional optical transceiver, a laser diode (LD, hereinafter) for generating the signal light and a photodiode (PD, hereinafter) for receiving the same are integrated with the same optical module. In the optical module in which transmission of the signal light and reception of the same is simultaneously performed, although a part of the signal light emitted from the LD is optically coupled with the optical waveguide, the remaining signal light leaks to the PD as a stray light. The stray light becomes a cause of a noise for the receiving signal light, and threreby a sensitivity in the receiving of the optical module deteriorates.
In the optical module in which transmission of the signal light and reception of the same are alternately performed by the time division multiplexing technology, when the stray light leaking from the LD reaches regions of the PD which are other than a light-receiving surface thereof, since a life time of carriers generated inside the PD is longer than that of the carriers generated on the light-receiving surface of the PD, a noise with a long time constant is caused in the PD and the sensitivity in receiving the signal light deteriorates just after the operation of the optical module is switched to the receiving mode from the transmitting mode.
In the conventional optical module for a bidirectional optical communication disclosed in Japanese Patent No.2579092, a light-shielding member provided with a pin hole is situated just before a PD, and thereby a light of a clad mode (an undesired light) propagating along a clad of a waveguide substrate or a stray light leaked from the semiconductor LD is prevented from being coupled with the PD. By making a light-receiving area of the PD larger than a cross-sectional area of a core of the optical waveguide and an area of the pin hole small in conformity with a cross-sectional dimension of the optical waveguide, the undesired light can be prevent form being couped with the PD.
As mentioned in the above, in the aforementioned conventional optical module, since the transmission path of the stray light leaked from the LD to the PD is complicated because of multiple reflections in a package and the optical waveguide, it is difficult to shield the stray light so as not to be coupled with the PD for receiving the signal light even in case that the pin hole is formed therefor.
Accordingly, it is an object of the invention to provide a structure for shielding a stray light in an optical waveguide module which a stray light leaked from a LD does not reach a PD for receiving a signal light to prevent a sensitivity in reception of a signal light from being deteriorated.
According to the first feature of the invention, a structure for shielding a stray light in an optical waveguide module comprises:
an optical waveguide substrate supported in an optical waveguide module package,
a LD which is mounted on the optical waveguide substrate and emits a first signal light,
an optical waveguide which is formed on the optical waveguide substrate and propagates a part of the first signal light,
a wavelength division multiplexing (WDM, hereinafter) filter which is formed on the optical waveguide substrate,
an optical fiber which is situated on the optical waveguide substrate and transmits the part of the first signal light to an optical transmission line,
a PD for receiving a second signal light which is propagated through the optical transmission line and transmitted through the WDM filter via the optical waveguide, and
a metallic layer which is evaporated on a surface of the WDM filter and provided with a pin hole for transmitting the second signal light to the PD via the WDM filter,
wherein a remaining part of the first signal light (a stray light, hereinafter) which is not optically coupled with the optical waveguide is reflected by the metallic layer so that the stray light is prevented from being coupled with the PD.
It is desirable to provide a light-absorption layer additionally evaporated on the metallic layer.
The light-absorption layer is formed of chromium oxide.
It is desirable that the WDM filter is situated on an end of the optical waveguide substrate.
It is desirable that the LD is situated on the side of the WDM filter.
According to the second feature of the invention, a structure for shielding a stray light in an optical waveguide module comprises:
an optical waveguide substrate supported in an optical waveguide module package,
a LD which is mounted on the optical waveguide substrate and emits a first signal light,
an optical waveguide which is formed on the optical waveguide substrate and propagates a part of the first signal light,
a WDM filter which is formed on the optical waveguide substrate,
an optical fiber which is situated on the optical waveguide substrate and transmits the part of the first signal light to an optical transmission line,
a photodiode for receiving a second signal light which is propagated through the optical transmission line and transmitted through the WDM filter via the optical waveguide, and
a plate which is stuck to a surface of the WDM filter, does not transmit the first and second signal lights but reflect them, and is provided with a pin hole for transmitting the second signal light to the PD via the WDM filter,
wherein a remaining part of the first signal light which is not optically coupled with the optical waveguide is reflected by the plate so that the stray light is prevented from being coupled with the PD.
As mentioned in the above, the invention especially relates to a structure for shielding a stray light used in an optical transceiver module using optical waveguides, and features of the invention can be summarized as follows.
(1) A filter on which a metallic layer having a pin hole is evaporated or a plate having a pin hole is stuck is inserted in a light path in a module.
(2) A stray light radiated from a clad of an optical waveguide is prevented from being coupled with a light-receiving device by the pin hole inserted in the light path.
(3) The metallic layer does not transmits the lights applied to the optical communication but reflects them.
(4) A size of the filter is larger than that of an end face of the optical waveguide substrate.
(5) Material forming the plate having the pin hole does not transmits the lights applied to the optical communication but reflect them.